The invention relates to an electrically actuatable vehicle brake and a method for controlling an electrically actuatable vehicle brake. The vehicle brake comprises a brake piston, which acts on at least one friction lining and can be displaced out of an initial position into an actuation position in which the brake piston abuts the friction lining against a rotating member of the vehicle brake which is to be connected, fixed against rotation, to a wheel of the motor vehicle, and a transmission unit, driven by an electric motor and acting on the brake piston, to actuate the brake piston, wherein the electric motor is to be triggered by an electronic control unit which is also set up to detect performance parameters of the vehicle, the electric motor and/or the vehicle brake. The invention further relates to an electrically actuatable vehicle brake, which is additionally fitted with a device for fixing the initial position of the brake piston, in which a predetermined, desired brake release play is kept constant.
Electrically actuatable vehicle brakes of this kind are known in various embodiments from the prior art. The forces occurring in these disc brakes as part of a braking process can be divided into clamping force (also called axial force, transverse force, brake application force or normal force) and peripheral force (also called frictional force).
The force component which is introduced into the brake disc by a brake shoe perpendicular to the plane of the brake disc is designated as clamping force. The peripheral force, on the other hand, is understood as the force component which acts on the brake shoe owing to the brake friction between a friction lining of the brake shoe and the brake disc in the peripheral direction of the brake disc. The braking moment can be estimated by multiplying the peripheral force by the distance of the point of application of the peripheral force from the axis of rotation of the wheels.
In a disc brake known from WO 99/05011, and corresponding U.S. Pat. No. 6,394,235, both of which are incorporated by reference herein the clamping force is generated either hydraulically or by means of an electric motor. In the case of hydraulic generation of clamping force a hydraulic fluid, put under pressure, is introduced into a hydraulic chamber constructed in a housing of the disc brake. A hollow piston of an actuator device accommodated displaceably in the hydraulic chamber is moved by the hydraulic fluid in the direction of one of the two brake shoes and brings it into frictional engagement with the brake disc. As the disc brake is configured as a floating caliper disc brake, the brake shoe which is not directly cooperating with the piston is also abutted against the brake disc in a known manner.
Where the clamping force is generated by an electric motor the rotational movement of a motor shaft is first reduced by means of a planetary gear train and then converted into a translatory movement by means of a nut and spindle arrangement of the actuator device arranged inside the hollow piston. The hollow piston is grasped by this translatory movement and transmits the translatory movement to one of the two brake shoes, which is thereupon abutted against the brake disc. In this vehicle brake, when the brake is not actuated a nut of the nut and spindle arrangement acting on the brake piston in the case of brake actuation by electric motor is arranged at a defined distance from the brake piston positioned in its initial position. In order to guarantee optimum functioning of the vehicle brake, this distance should be set as accurately as possible, as too great a distance leads to lengthening of the response time of the brake, while if the distance is too small a residual slipping moment could arise between the rotating brake disc and the friction linings.
In all known electrically actuatable disc brakes the fundamental problem arises that the distance between the friction linings and the brake disc when the brake is not actuated, the so-called brake release play, increases as a result of wear of the friction linings, thereby lengthening the response time of the brake. There is a further problem with brake arrangements of the kind known from WO 99/05011, in which the initial position of the brake piston is fixed not with the aid of an element of the transmission unit acting on the piston, but by means of a separate device, that the defined distance between the component of the actuator device acting on the brake piston in the case of brake actuation by electric motor and the brake piston can become undesirably smaller or larger in the course of the operating life of the vehicle brake.
A method for controlling or regulating a brake is known from DE 197 30 094 A1, and corresponding U.S. Pat. No. 6,536,562, both of which are incorporated by reference herein, with a first friction surface electrically actuatable by means of an actuator and a second friction surface, between which a release play is provided. For this the position of the actuator and the current to be supplied to the actuator arc detected and abutment of the first friction surface to the second is fixed and thereupon a contact signal generated.
In order to enable recognition and setting of the release play using actuator-specific parameters independently of a special brake actuation and also to enable re-setting of the release play while the vehicle is travelling, to fix the abutment the change in the actuator current and the change in the actuator position are evaluated.
From DE 198 35 550 A1 a motor vehicle disc brake device is known, with a brake housing, a brake piston axially movable hydraulically/mechanically in the brake housing and further with a hydraulic actuating device and with a mechanical actuating device with a rotatable spindle and a nut which is axially displaceable in respect of the spindle by rotation thereof. The brake piston can be impacted hydraulically by means of the hydraulic actuating device and/or mechanically by means of the mechanical actuating device. The brake piston is supported on the nut in the case of mechanical actuation.
In order to configure this arrangement as more independent of the effects of temperature and ageing, at least one sensor is provided in this motor vehicle disc brake device, wherein the mechanical actuating device correspondingly tracks the brake piston as a function of brake lining wear detected by the sensor and wherein the mechanical actuating device fixes the brake piston if an appropriate brake is established by hydraulic methods and detected by the sensor.
Underlying the present invention is the problem of increasing the operational safety of an electrically actuatable disc brake and in particular guaranteeing short response times of the brake in braking operation.
To solve this problem, according to the invention an electrically actuatable vehicle brake with the features cited in the independent device claims and a method for controlling an electrically actuatable vehicle brake according to the independent method claims is proposed.
The electrically actuatable vehicle brake comprises a brake piston, acting on at least one friction lining and displaceable out of an initial position into an actuation position in which the brake piston abuts the friction lining against a rotating member of the vehicle brake to be connected, fixed against rotation, to a wheel of the motor vehicle, and a transmission unit, driven by an electric motor and acting on the brake piston, to actuate the brake piston, wherein the electric motor is to be triggered by an electronic control unit which is also set up to detect performance parameters of the vehicle, the electric motor and/or the vehicle brake. The electronic control unit is set up and programmed to trigger the electric motor by means of a control and evaluation program running in it in such a way that the brake piston is displaced in response to the detecting of at least one predetermined operating condition in a first direction into its actuation position, in which the at least one friction lining abuts against the rotating member, and the brake piston is then displaced by a path corresponding to a predetermined, desired brake release play in a second direction, opposite the first direction, into its initial position.
By regularly carrying out a calibration of this kind of the brake arrangement according to the invention the wear of the friction linings can be reliably compensated for and the brake release play kept at a constant value. The frequency with which the calibration is carried out can be set as wished by suitable selection of the operating condition to be detected and depends, for example, on the demands made on the system by functions such as ABS, ASR, EPB, etc., for example.
If the vehicle brake according to the invention additionally comprises a device for fixing the initial position of the brake piston, which fixes the initial position of the brake piston in such a way that a predetermined brake release play is kept constant during the operating life of the brake, the electronic control unit is set up and programmed by means of a control and evaluation program running in it to trigger the electric motor in such a way that, in response to the detecting of at least one predetermined operating condition, an element of the transmission unit acting on the brake piston where there is electric actuation of the brake is displaced in a first direction into its actuation position, in which the brake piston actuated by the element abuts the at least one friction lining against the rotating member. The element is then displaced in a second direction opposite the first direction, into its initial position, by a path corresponding to the predetermined brake release play and, if wished, a predetermined distance between the element and the brake piston when the brake is not actuated.
If the initial position of the brake piston in an electrically actuated brake of this kind, which can be configured, for example, in such a way that the brake piston can be actuated both hydraulically and by electric motor, is established by a separate device, with the aid of calibration of this kind the distance existing when the brake is not actuated between the element of the transmission unit acting on the brake piston in the case of electric brake actuation and the brake piston can be kept at the desired value.
Modern motor vehicles are fitted with a bus system (CAN, for example), to provide estimated performance parameters of components or modules of the motor vehicle to the control units present in the vehicle or to communicate trigger signals from the control units to components or modules of the vehicle. Therefore performance parameters of the vehicle, the electric motor and/or the vehicle brake (for example, operating state of the vehicle, current consumption of the electric motor and braking force exerted by the vehicle brake) are usually detected and evaluated for other purposes in any case. Thus no further outlay on components, circuitry, etc. arises for detecting the predetermined operating condition or for detecting the actuation position of the brake piston. Thus the increased operational safety resulting from the invention and the short response time of the braking system can be provided with the smallest additional outlay. However, if the performance parameters of the vehicle, the electric motor and/or the vehicle brake required for the calibration of the braking system are not available in the bus system, it is necessary to supply these data to the electronic control unit (ECU) in some other way (by suitable sensors).
In a particularly preferred embodiment of the electrically actuatable vehicle brake without the device for fixing the initial position of the brake piston the electronic control unit is set up to trigger the electric motor in such a way that the brake piston is displaced into a predetermined zero position before being displaced into its actuation position. Moreover, the electronic control unit is set up to detect a dimension which is correlated to a path covered by the brake piston between its zero position and its actuation position and to compare the detected dimension with a stored reference dimension, the reference dimension being correlated to a path covered by the brake piston in a vehicle brake fitted with new friction linings.
Because of the wear of the friction linings and the resulting enlargement of the brake release play the path to be covered by the brake piston from the zero position to the actuation position lengthens in comparison to a path covered by the brake piston in a vehicle brake with new, unworn brake linings. The comparison between the detected dimension, which is correlated to the path covered by the brake piston, and the reference dimension, which is correlated to the path covered by the brake piston in a vehicle brake fitted with new friction linings, therefore enables determination of the overall wear of the friction linings. To carry out the above-described functions the electronic control unit may have a computer unit equipped with a memory, which calculates from the detected dimension and the reference dimension stored in the memory a difference corresponding to the wear of the friction linings. A suitable dimension correlated to the path covered by the brake piston is, for example, the number of steps of the electric motor. The brake arrangement according to the invention therefore preferably comprises a sensor, which detects the steps of the electric motor. The value output by the sensor can then be set at zero in the electronic control unit when the brake piston is in its zero position.
The electronic control unit is preferably set up to issue a warning signal to the vehicle driver if a difference between the detected dimension and the reference dimension exceeds a predetermined value. Additionally or instead of this a quantitative indication (for example, by means of a bar display) of the thickness of the brake lining can also be given to the driver. The predetermined value may, for example, correspond to a critical wear value of the friction linings at which a change of friction linings should be made. The signal issued to the driver may be an acoustic or optical signal, issued, for example, at the start of a journey when the ignition is switched on.
In a similar way the electronic control unit in the vehicle brake fitted with the device for fixing the initial position of the brake piston can be set up to trigger the electric motor in such a way that the element of the transmission unit acting on the brake piston is displaced into a predetermined zero position before being displaced into its actuation position. The electronic control unit is then further set up to detect a dimension which is correlated to a path covered by the element between its zero position and its actuation position and to compare the detected dimension with a stored reference dimension, the reference dimension being correlated to a path covered by the element in a vehicle brake fitted with new friction linings.
Like the path of the brake piston, the path covered by the element acting on the brake piston from the zero position to the actuation position also lengthens as a function of the wear of the friction linings. If the initial position of the brake piston is fixed by a separate device, for example a “rollback” seal cooperating with the brake piston, a comparison of the measured dimension correlated to the path covered by the element with a reference dimension which is correlated to the path covered by the element in a vehicle brake fitted with new friction linings can also be drawn on to determine the overall wear of the friction linings. The electronic control unit then preferably has a computer unit equipped with a memory, to calculate a difference, corresponding to the wear of the friction linings, between the detected dimension and the reference dimension stored in the memory. A suitable dimension correlated to the path covered by the element is, for example, the number of steps of the electric motor. Therefore there is preferably a sensor which detects the steps of the electric motor. The value output by the sensor can then be set at zero in the electronic control unit when the element acting on the brake piston is in its zero position.
The effect caused by the “rollback seal” and the configuration of a seal of this kind leading to it is described in DE 196 47 434 A1, and corresponding U.S. Pat. Nos. 6,164,422 and 6,439,351, all of which are incorporated by reference herein, for example. On braking the piston moves by hydraulic pressure in the direction of the brake disc and the elastomer sealing ring is slightly entrained by the displacing piston and thereby also slightly deformed by the hydraulic pressure.
When the pressure on the brake is relieved, i.e. when the hydraulic pressure ceases, the deformed elastomer sealing ring exerts a restoring force on the piston and entrains it a small amount in the direction away from the brake disc. This behaviour is totally desirable, because it resets the brake piston and ensures that the so-called brake release play sets in between the friction lining and the brake disc after braking has ended. Reference is made on this to German patent specification 1 600 008, and corresponding U.S. Pat. No. 3,421,604, both of which are incorporated by reference herein, in which this behaviour is described in detail. To support the previously described behaviour of the sealing ring, which is preferably made of an elastomer, even the side wall on the brake lining side of the groove in which the sealing ring is held is frequently chamfered, i.e. the groove cross-section widens from the bottom of the groove outwards (see on this FR PS 1 504 679, and corresponding U.S. Pat. No. 3,377,076, both of which are incorporated by reference herein).
A certain adhesion between the cooperating surfaces of the sealing ring and the brake piston is required for the sealing ring to be able to reset the brake piston. If the sliding properties between these surfaces were to be too good this would prevent re-setting of the piston, as the latter would be displaced by sliding relative to the sealing ring virtually immediately, which would mean there would be no longer be any deformation of the sealing ring and therefore no restoring force either.
On the other hand, under certain circumstances sliding of the brake piston relative to the sealing ring is very desirable. This is the case, for example, when the brake is being assembled. Here the brake piston has to be pushed into the cylinder after the sealing ring has been placed into the groove accommodating it in the cylinder wall. If the piston and the sealing ring were not able to slide relative to one another, it would be very probable that the sealing ring would twist in the groove or would be damaged and partially sheared off.
Sliding of the brake piston relative to the sealing ring is also necessary, however, in certain operating states of the brake, for example if the thickness of the friction lining is reduced by wear, so the brake piston has to be displaced correspondingly further in the direction of the brake disc to abut the friction lining against the brake disc. Under such circumstances the deformability of the elastomer sealing ring is exceeded and the piston has to slide relative to the sealing ring to adopt a new position relative to the sealing ring.
Sliding of the brake piston relative to the sealing ring may also become necessary with very strong braking operations, as, owing to the large forces, the brake housing then widens out, the material of the friction lining is compressed, etc., so a correspondingly larger displacement path of the brake piston has to be produced.
The electronic control unit can, moreover, be set up to issue a warning signal to the vehicle-driver if a difference between the detected dimension correlated to the path covered by the element from the zero position to the actuation position and the stored reference dimension correlated to the path covered by the element in a vehicle brake fitted with new friction linings exceeds a predetermined value. The predetermined value preferably corresponds to a critical wear value of the friction linings at which a change of friction linings should take place.